Removal of organic and mineral deposits from solid surfaces has been accomplished in the past with limited success. This, of course, includes removal of organic and mineral deposits from semipermeable membranes which are used in reverse osmosis systems to purify water and from ion-selective membranes used in electrodialysis systems.
Osmosis concept is based on the use of a semipermeable membrane which is semipermeable to water but which rejects certain dissolved salts. If pure water is separated from a salt solution by a semipermeable membrane, water will flow through the membrane from the pure water side to the impure water side or from dilute solution side to the more concentrated solution. This results in diluting the more concentrated solution and such process continues until osmotic equilibrium is reached at which point, osmotic pressure or solution concentration on both sides of the membrane is about equal.
It is known, however, that if positive pressure is applied to the salt solution sufficient to overcome the osmotic pressure, the flow will be reversed and water will flow from the salt solution through the membrane to the pure water side. This is what is meant by reverse osmosis. To accomplish this, pressure of 600 to 800 psi is usually applied to the feedwater side in order to reverse the flow of water to the pure water or product water side. Product water is 95 to 99% free of dissolved material. Such pressures are generally used to purify saline water by forcing the water through the membrane which rejects minerals.
Electrodialysis is an electrochemical separation process in which salts that are dissolved in water are forced through ion selective membranes under the influence of an applied electric field. The net result of this dialytic process is the transfer of ions from a less concentrated solution to a more concentrated solution. Electrodialysis reversal is simply an electrodialysis process in which the polarity of the applied direct current potential is automatically reversed at regular 15 to 30 minute intervals. Polarity reversal changes the direction of ion movement within the membrane stack.
Operational limitations on unidirectional membrane processes are imposed by the chemistry of the concentrate or brine stream. Long term, stable system performance is of critical importance for industrial operations and municipal supplies. Membrane fouling and mineral scale formation radically degrade system performance. Typical pretreatment for unidirectional membrane processes includes presoftening or treatment of the feedwater with acid and/or complexing agents such as polyphosphates. Such pretreatment and chemical feed requirements add the burdens of cost and waste treatment to the desalting process.
The semipermeable membranes are generally thin and delicate. They can be supported on spongy or foamed matrix to provide mechanical support for the membrane. Such matrices have open cells which allow passage of water or liquid. Semipermeable membranes are made from any suitable material such as cellulose acetate, cellulose triacetate, a polyamide, or a polysulfone.
The continued efficiency of a reverse osmosis system depends on the maintenance of the membrane in an unfouled condition. Probably the greatest problem experienced in the use of these systems is fouling of the membrane by scale. Typically, the membrane becomes fouled by scale build-up to a point where it must be replaced quite often. The cartridge containing the membrane must be removed and replaced by a clean cartridge. The used cartridge is then treated to remove scale. Obviously, it is desirable to prevent scale build-up or at least, prolong the time between cartridge changes. This is ordinarily done by injecting certain chemical additives to the impure water, which are used for the purpose of preventing the build-up of foulants.
Cleaning of the membrane can be made in place whereby the piping is provided to allow for recirculation of a cleaner solution. In this fashion, valves are manipulated to allow for recirculation of the cleaner solution through the membrane until the membrane is cleaned to the point where it can be returned into a reverse osmosis system. In some commercially operating systems, a membrane cartridge is removed and placed in a cleaner mode where a cleaner solution is recirculated through the membrane in the cartridge until the membrane is sufficiently clean for reuse. In either case, a cleaning solution is prepared which is capable of removing scale and other foulants from the membrane. Also, in some situations, an additive can be added to infeed water to prevent or reduce formation of foulants which, otherwise, would deposit on the membrane and thus clog it.
U.S. Pat. No. 4,357,254 describes various prior art which generally relates to removal of calcium and magnesium scale. That patent itself is directed to compositions for cleaning solid surfaces and reverse osmosis membranes for removal of calcium, magnesium and iron scale. The iron scale is generally iron oxide which is primarily formed by the use of steel pipes or fittings which gradually raise the level of ferrous iron in water. The ferrous iron is then oxidized by dissolved oxygen to form ferric iron which hydrolyzes to ferric oxide or hydroxide which deposits on the membrane. Ferrous ion can also enter via the feedwater.
For cleaning reverse osmosis membranes containing little or no iron scale, U.S. Pat. No. 4,357,254 discloses compositions comprising a monobasic or dibasic sodium phosphate, citric acid, malic acid, and a nonionic surfactant. Citric and/or malic acids are used in amount of at least 20%. That patent also discloses that in reverse osmosis systems where there is a significant amount of iron oxide scale, oxalic acid should also be included. Such compositions are effective for cleaning fouled reverse osmosis membranes by dissolving and dispersing organic and mineral deposits, which primarily are calcium and magnesium scale as well as silicates and colloidal clay.
As is noted in U.S. Pat. No. 4,357,254, the disclosed composition is dissolved in water to a concentration of from about 0.1% by weiqht to about 5% by weight. Any concentration within that range will effectively clean the membrane, the primary effect of varying concentration being in the cleaning time. At a preferred concentration of about 2%, the average system can be cleaned in from 1/2 to 1 hour at about 25.degree. C. As the concentration goes to the higher end of the range, the saving in cleaning time, due to the kinetics of the system, is not appreciably shortened over the 1/2 to 1 hour cleaning time at the preferred concentration of 2%. As the concentration goes to the low end of the range, the cleaning time can become inconveniently long. Even at the low end of the range, however, the capacity of the solution is more than adequate to effectively clean the membrane.
By means of another approach, the composition can be injected into the input water to provide 0.01 to 5000 ppm, preferably 0.1 to 50 ppm, for the purpose of maintaining the membrane in a relatively clean condition or to prolong the use of the membrane by keeping it cleaner longer. Pursuant to this approach, membranes are kept in operation for extended periods before they are cleaned or replaced.
U.S. Pat. No. 4,386,005 discloses the synergistic relationship of a low molecular weight polyacrylic acid and phytic acid compositions to reduce build-up of calcium, magnesium, and/or iron scale. In col. 3, this patent discloses pertinent prior art and the unique feature of iron scale which can damage the membrane by growth of crystals within the membrane.
U.S. Pat. No. 4,496,470, which is a c-i-p of U.S. Pat. No. 4,357,254, describes similar semipermeable membrane cleaner compositions as U.S. Pat. No. 4,357,254 but additionally discloses that sulfamic acid can be used in place of or in conjunction with citric acid and/or malic acid, i.e., weak organic acids. Sulfamic acid is also a weak organic acid.
U.S. Pat. No. 4,386,005 describes scale-inhibiting compositions of low molecular weight polyacrylic acids which are effective against calcium and magnesium scale to a point where cleaning of a semipermeable membrane is not necessary for several months. This patent also discloses that phytic acid is at least as effective as low molecular weight polyacrylic acid for inhibiting calcium and magnesium scale build-up on reverse osmosis membranes. Phytic acid is also very effective in inhibiting formation or deposition of iron scale on a reverse osmosis membrane where the feedwater is saline or brackish. Although saline water has a low concentration of iron, brackish water has an extremely high iron concentration.